AT&T Gophone GEMINI Compatible Battery 3.7V 800mAh

AT&T Gophone GEMINI / OT871AG — 3.7V Li-ion Replacement Battery

This 3.7V 800mAh Li-ion cell is a direct replacement for the AT&T Gophone GEMINI and OT871AG. It fits the original battery compartment and connects to the same charge circuit used in both variants. No OEM part number is published for this cell, but the physical dimensions — 49.34 × 40.00 × 4.45mm — match the stock battery slot exactly.

• GEMINI and OT871AG compatibility: Both devices share the same battery bay dimensions and 3.7V single-cell architecture. The charge management IC on each board accepts the same cutoff voltage, so one cell fits both without modification.
• Bench tested on actual hardware: We ran a full charge-discharge cycle and confirmed the BMS held charge termination at 4.2V and triggered low-voltage cutoff cleanly. No thermal spikes during the CC-CV transition.
• Fuel gauge recalibration on first use: On first use after installation, run one full discharge down to auto-shutoff, then charge uninterrupted to 100%. This gives the fuel gauge IC a complete reference curve for the new cell and prevents erratic percentage readings from the first day.

Sudden shutdown at 20–30% on the Gophone GEMINI

This is a voltage cliff issue, not a capacity problem. When the modem transmits or the screen runs at full brightness, current draw spikes and the cell voltage drops sharply below the BMS cutoff threshold — even if the fuel gauge still shows charge remaining. An aged or freshly installed uncalibrated cell hits this cliff earlier than expected. Run one full discharge-charge cycle and let the fuel gauge IC map the new cell's actual discharge curve. After calibration, the reported percentage and the real cutoff point align closer to 5–8% rather than 25–30%.

Phone won't power on after sitting in storage

Li-ion cells left uncharged below roughly 2.5V trigger a BMS lockout state — the battery stops responding to a standard charge handshake. The phone sees no valid cell voltage and won't boot. Connect the phone to a wall adapter, not a USB port on a computer, and leave it for 20–30 minutes without pressing the power button. The charge IC on the phone board sends a low-current recovery pulse that brings the cell back above the 2.5V re-enable threshold, after which normal charging resumes.